In a manufacturing process of various semiconductor devices, a silicon oxide film, such as SiO2, is formed as an insulating film, e.g., a gate insulating film of a transistor. In order to form such a silicon oxide film, a thermal oxidation process using an oxidation furnace or a rapid thermal process (RTP) apparatus is used. In a wet oxidation process using an oxidation furnace, which is one of the thermal oxidation processes, a silicon substrate is heated to a temperature exceeding 800° C. and exposed to an oxidizing atmosphere of water vapor (H2O) by using a water vapor generator (WVG), which generates vapor (H2O) through the combustion of oxygen and hydrogen, thereby oxidizing a surface of the silicon substrate to form a silicon oxide film.
The thermal oxidation process is considered as a process of forming a silicon oxide film of a good quality. However, the thermal oxidation process requires a high temperature exceeding 800° C., and thus causes problems, such as the increase of a thermal budget, the distortion of a silicon substrate due to thermal stress, or the like.
On the other hand, there is proposed an oxide film forming method as a technique capable of avoiding the increase of the thermal budget or the distortion of the silicon substrate in the thermal oxidation process (see, e.g., WO2001/69673). In this method, an oxidation process is performed on the surface of an electronic device mainly containing silicon by using a microwave-excited plasma, which is formed at a pressure of 133.3 Pa in a chamber using a processing gas including Ar gas and oxygen gas, the proportion of the flow rate of oxygen in the processing gas being approximately 1%. Accordingly, it is possible to form a silicon oxide film having a good quality and easily controlled film thickness.
In case that the plasma process is carried out under the condition that the process pressure is approximately 133.3 Pa and the proportion of the flow rate of O2 in the processing gas is 1% (for convenience of description, referred to as a low-pressure and low-oxygen concentration condition), for example, when a pattern, such as lines and spaces formed on an object to be processed, has dense and sparse portions, there is a difference of forming speeds of the silicon oxide film between dense portions and sparse portions, and it is difficult to form the silicon oxide film with a uniform thickness. If the thickness of the silicon oxide film varies according to the portions of the film, the reliability of a semiconductor device using the silicon oxide film as an insulating film may be lowered.
In order to solve this problem, the plasma oxidation process is carried out under the condition that the process pressure is approximately 667 Pa and the proportion of the flow rate of O2 in the processing gas is approximately 25% (for convenience of description, referred to as a high-pressure and high-oxygen concentration condition). In this case, however, when a silicon oxide film is formed on a pattern having prominences and depressions, an oxidation rate at a dense portion is lowered, and corners of the upper ends of the prominences are not sufficiently rounded. Thus, leakage current due to electric field concentration on these portions or cracks due to stress of the silicon oxide film may be generated.
That is, in case that a silicon oxide film is formed by the plasma oxidation process, it is required to round corners of the upper ends of prominences of the pattern and also to form the silicon oxide film having a uniform film thickness regardless of the density of the pattern.
Further, an oxide film requires two electrical characteristics, i.e., a time zero dielectric breakdown (TZDB) characteristic and a time-dependent dielectric breakdown (TDDB) characteristic, and it is required that both these insulating properties are good.